Numerous multimedia application services, such as Video-On-Demand, television broadcast, video conferencing, online education, and interactive games, have emerged along with rapid development of mobile communication and the Internet, and a plurality of users need to receive the same data simultaneously. These mobile multimedia services are characterized by a large amount of data, a long duration, etc., as compared with general service. For effective utilization of mobile network resources, the Third Generation Mobile Communication Standardization organization (i.e. 3rd Generation Partnership Project (3GPP)) has proposed the Multimedia Broadcast/Multicast Service (MBMS). The MBMS refers to a point-to-multipoint service for transmitting data to a plurality of users from one data source, which enables sharing of network resources including mobile core network and access network resources, particularly air interface resources. The MBMS is different from the existing Cell Broadcast Service (CBS) in a mobile network. The CBS is a message-based service which allows low bit rate data to be transmitted to all users via a cell shared broadcast channel. The MBMS defined by the 3GPP can accomplish not only the multicast and broadcast of plain-text and low rate messages but also the multicast and broadcast of high rate multimedia services, e.g., a mobile TV service.
To efficiently utilize radio resources, network sides in broadcast and multicast modes typically transfer information to multiple User Equipments (UEs) using a common wireless channel. At present, it is defined in a 3GPP protocol that a transmission channel bearing point-to-multipoint transfer in the MBMS is a Forward Access Channel (FACH) and a corresponding physical channel is a Secondary Common Control Physical Channel (S-CCPCH). Because the Multimedia Broadcast/Multicast Service has a large amount of data and reception objects positioned in multiple points at uncertain positions, the same information needs to be transmitted by multiple network equipments through omnidirectional transmission at relatively large power, so that the whole cell can be covered by the transmitted information. In the case of a single frequency operation, a relatively large interference may occur between adjacent cells supporting the MBMS, so that performance of the system is reduced and the application of the Broadcast/Multicast Service is constrained.
In the existing MBMS protocols, selective combining and soft combining modes for the point-to-multipoint transmission are also defined to reduce the interference between the adjacent cells. The selective combining is achieved by the counting of a Radio Link Controller Protocol Data Unit (RLC PDU). That is, when MBMS wireless bearer rates of the adjacent cells are close to each other, and MBMS data streams from different cells do not exceed capability of reordering of a User Equipment Radio Link Controller (UE RLC), the UE may perform the selective combining. The soft combining mode requires a physical layer of a terminal to be capable of combining bit data from physical channels of different wireless links. For example, the terminal combines S-CCPCHs of two network equipments. To achieve the combining, the different S-CCPCHs need to adopt the same transmission format combination and the same data field, and the network side needs to determine cells that may be combined and directly notify the terminal about wireless frames of the S-CCPCHs that need to be combined.
However, in an existing Time Division Duplex (TDD) system, there is no requirement of macro diversity for the UE. The so-called macro diversity means that the UE maintains communicating with two or more network equipments simultaneously, so as to improve quality of the received signal. Therefore, a UE receiver with general design cannot meet the requirement of the above mentioned selective combining and soft combining for the MBMS.
Moreover, in a wireless frame structure of an existing Time Division Synchronous Code Division Multiple Access (TD-SCDMA) system as shown in FIG. 1, each wireless subframe has duration of 5 ms and occupies 6400 chips in total, and is composed of 7 normal service time slots and 3 special time slots. As shown in FIG. 2, burst structure parameters of a normal service time slot include a training sequence (a channel estimation code) for channel estimation positioned in the middle of the burst structure, data symbols for transmitting service data and positioned at the both sides of the burst structure, and a Guard Period (GP) which is a duration extension for guarding the next time slot. In FIGS. 1 and 2, duration Tc of one chip is 0.781 μs, duration Tts of one time slot is 675 μs, duration T of the channel estimation is 0.781×144=112.5 μs, and duration Tgp of the Guard Period is 0.781×16=12.5
The capability of receiving multipath of the MBMS (that is, the capability of receiving multipath energy from multiple cells) depends on a size of an extension cycle window in the structure of the channel estimation code. If a signal falls outside the extension cycle window, the existing channel estimation based on a Fast Fourier Transform (FFT) method cannot be achieved. Since it is required that the multipath of the MBMS does not interfere with other service time slots, a size of the Guard Period in the time slot structure also constrains the capability of receiving the multipath of the MBMS. That is, the capability of receiving the multipath of the MBMS depends on the two parameters of the extension cycle window and the Guard Period of the channel estimation code. In the existing TD-SCDMA system, the values of these two parameters each are 16 chips, which is equivalent to a transmission distance of d=12.5×10-6×3×108=3750 m=3.75 km. Such a transmission distance indicates that a signal from a cell beyond 3.75 km does not fall within a valid multipath reception window, instead brings about interference. Further, a radius of a cell is limited to be within 3.75 km, so that the coverage of the cell is reduced.
In view of the above, based on the structure of a wireless frame of the existing TD-SCDMA system, a scheme of time diversity combining and a scheme of macro diversity based on joint detection are proposed for the TDD system. In the method of time diversity combining, the time when the same information is transmitted from different cells is controlled so that the information arrives at the terminal at different time slots, accordingly, the UE sequentially processes the information from the different cells at the different time slots and then combines the processed information, thereby avoiding the requirement for the UE to perform reception from multiple links simultaneously. To realize the time diversity combining, the cells bearing the MBMS are divided into different sets, each of which is assigned to a unique time slot or time slot combination, and the time slots or time slot combinations do not overlap in time. When a set of cells transmits information at the assigned time slot, other sets of cells are in an inactivated state. Data is received by the UE from the different sets of cells, and combined at the physical layer or an RLC layer. As shown in FIG. 3, three time slots (t1, t2, and t3) are assigned for the MBMS transmission. The same signals from different cells are received by the UE at the three time slots t1, t2 and t3, respectively, and are combined at the physical layer or the RLC layer to improve the reception performance. In the scheme of time diversity proposed for the TDD system, the interference between the adjacent cells is reduced at the expense of channel capacity. For example, the above three time slots t1, t2 and t3 are assigned to three MBMS sets, respectively. When the time slot t1 is used by a set 1, data cannot be transmitted from sets 2 and 3 at the time slot t1, which wastes the system resources greatly. Such scheme satisfies the MBMS service performance at the price of an increase of the consumed system resources, and therefore is not adopted in the present product application implementation of the MBMS. As for the scheme of macro diversity based on joint detection, both the limited capability of the joint detection of a terminal (which supports joint detection of only 3-4 cells at present) and broadcast service limitation due to the structure of the wireless frame of the existing TD-SCDMA system limit the number of cells of the macro diversity based on the joint detection method at the terminal, thus the macro diversity combining of multi-cells and the macro cell coverage for the MBMS cannot be efficiently achieved.
Further, in a Long Term Evolution (LTE) system, the 3GPP organization adopts a modulation mode of Orthogonal Frequency Division Multiplex (OFDM) in the evolution design of a 3G system (Frequency division duplex (FDD) and Time division duplex (TDD)), where a macro diversity of a Single Frequency Network (SFN) mode is adopted for the evolution of the MBMS technology and a frame structure with a long Cyclic Period (CP) is adopted to implement the SFN macro diversity technology.
The implementation scheme of the existing SFN technology is as follows: the same temporal resources, frequency resources, scrambling code and channel estimation code used for a broadcast service are collectively allocated for all cells in the SFN network, and the temporal resources, the frequency resources, the scrambling code and the channel estimation code are also used by a UE in a cell to receive the broadcast service, that is, as long as a signal from a cell in the SFN network falls within the window of a multipath receiver of the UE, the UE may integrate energies of all such signals falling within the reception window directly at an air interface to thereby improve greatly reception performance of the broadcast service. The purpose of the design of the frame structure with a long CP is to expand the multipath reception window of the UE, to enable the UE to receive a broadcast signal from a farther cell.
To sum up, the diversity technology adopted based on the frame structure of the existing TDD TD-SCDMA system can efficiently implement neither the macro cell coverage of the Broadcast/Multicast Service, nor the macro diversity combining of more cells.